Ceramic recuperators for industrial waste heat recovery have several advantages over conventional metallic recuperators. For example, ceramics in general have high corrosion resistance, high mechanical strength at elevated temperatures, low thermal expansion coefficients (TEC'S) and good thermal shock resistance and thus exhibit excellent endurance under thermal cycling; are light in weight (about 1/3 the weight of stainless steel); and are cost competitive with high temperature alloys. Furthermore, ceramic recuperators are available in a variety of shapes, sizes, hydraulic diameters, (hydraulic diameter is a measure of cross-sectional area divided by wetted perimeter) and compositions.
To render such ceramic recuperators compatible with existing furnace, oven and preheater structures, special housings have been designed.
In U.S. Pat. No. 4,083,400, issued Apr. 11, 1978 and assigned to the present assignee, a ceramic cross-flow recuperator core is incorporated into a metallic housing adapted for retrofitting to the metallic fittings of existing furnaces, ovens and preheaters. Insulating and resilient sealing layers between the core and housing minimize heat loss through the metallic housing and prevent leakage of heat transfer fluids, such as exhaust flue gasses and incoming combustion air, past the core.
In U.S. patent application Ser. No. 951,438, filed Oct. 16, 1978 and assigned to the present assignee, a housing for a ceramic flow recuperator comprises two pairs of opposing apertured plates with means for maintaining the plates in firm contact with the inlet and outlet faces of the ceramic recuperator. These plates, as well as the ceramic faces, may easily be machined to close-tolerance flat surfaces for optimum sealing contact, thus enabling minimization of gas leakage past the ceramic-metal seal. Metal conduits extend a short distance from the plates' external surfaces opposite the contact surfaces, and are adapted for connection to heat transfer fluid conduits.
In both of the above designs, the conduit portions are generally tapered inwardly in a direction away from the housing to the point of connection with the external fluid conduits in order to coincide with the somewhat smaller cross-sections of such conduits as compared to the ceramic recuperator faces. Such tapering requires greater conduit wall area than would a cylindrical design, and thus leads to greater through-wall heat loss.